Squamous Cell Carcinoma

Last Updated on March 31, 2022 by Allison Price

Primary squamous-cell carcinoma is a common form of cancer in horses. It is only found in areas with squamous cell carcinoma, which can be the skin, stomach, and nasal cavities.
The appearance of the tumour under a microscope can help determine how malignant it is. It is considered less aggressive if the cells look normal and produce keratin. High-grade malignant cells produce very little keratin and can spread quickly via blood and lymphatics, to the local lymph node (gland), or to internal organs.

Carcinoma: What Causes It?

Exposure to carcinogens (cancer-inducing agents) can cause carcinoma. The risk is higher in areas where the sun shines more (closer to the Equator). This risk factor is comparable to human skin cancer.

Different types of Carcinoma

There are three clinical forms.

  1. Proliferative: This is the common growth found on older geldings’ penis and on the cornea (surface) of the eyes.
  2. Ulcerative/Destructive: The tumour results in tissue loss, erosions and ulcerations. These types are most common around the perineal or anus area, and on the third and non-pigmented eyelids.
  3. Combined ulcerative/Destructive forms: Quite a common form which includes the vulvar or clitoral forms. This type is more common in younger horses.

Squamous cell carcinoma of Eye

Squamous cell carcinoma is found in the eyelids, third eyelid, outside corner (laterallimbus) and cornea.
Eyelid squamous cells carcinoma can be very destructive and invasive.
Non-pigmented eyes are more likely to develop the ulcerative and destructive form.
A lateral limbal proliferative form and a carcinoma of the corneal epithelium are two of the most common ocular forms. The corneal form is characterized by a cloudy or pus-like discharge.
It is easier to diagnose carcinoma when it is advanced. The first sign is often excessive tear production. A urine blood dipstick can usually detect blood in the tears. Sometimes, the tears can be visible blood stained.

Cutaneous (Skin) Squamous Cell Carcinoma

Cutaneous forms squamous cells carcinoma can be found in the face, ears, and perineal region. The majority of these are ulcerative/destructive forms. Prognosis will depend on the extent and location of the tissue loss.

Penile Squamous Cell Carcinoma

Penile squamous-cell carcinoma can be divided into two main types.

  • The most common form is found in older horses. Stallions are less likely to be affected. This suggests that smegma could be a contributing factor. The lesions may be proliferative or ulcerative/destructive or there may be a combination of the two. Most often, the glans (head) of the penis is affected. However, the shaft of penis skin may also be affected. Multiple lesions can occur and they may become proliferative or ulcerated.
  • Younger horses (ages 10-14 years) have a higher rate of malignancy. These cases are often too advanced by the time they present at the vet. The primary lesion is often the glans (head or penis).
Squamous Cell Carcinoma

Some lesions, especially those with a more severe or chronic course, may “seed” into skin at the prepuce. These lesions can be very severe and may not occur with the penile ones. These cases are very difficult to treat. The only way to get rid of these cases is to remove the entire penis and prepuce. This site may become infected and cause scarring. Urinating difficulties can be the first sign of this condition.

Pre-cancerous changes in the skin and glans of horses may be indicated by preputial discharge.

Vulvar and Clitoral Squamous-Cell Carcinoma

Vulvar, or the clitoral form, can develop in older mares. It can also occur on skin with pigmented. The clitoral form may develop due to the carcinogenic effects of smegma. Vulvar carcinomas usually become proliferative within the vestibule and then turn into ulcerative when they reach the lips of the vasa. Proliferative is the clitoral form. Radiation and surgical excision are often used in combination.

Mucosal Squamous Cell Carcinoma

The mucosal surface of the throat, nose, sinuses, and mouth (pharynx) is made up of squamous cells. Chronic blood staining or pus, such as nasal discharge or bleeding, are the main signs. Secondary signs include nasal obstruction, difficulty swallowing or eating (dysphagia), and nasal obstruction. However, early clinical signs might not be apparent. These forms can cause facial distortions and swellings, which are often proliferative. Squamous Cell Carcinoma Mucosal Forms often have an aggressive destructive nature. This can lead to large-scale proliferative lesion and extensive destruction.

Gastric Carcinoma

Only the oesophageal portion of the stomach is affected by gastric squamous cells carcinoma. Habronema and Draschia species can cause recurrent gastric infections. Cancerous changes may be caused by Bots or worms. High levels of any of these parasites in horses are not likely to develop any diseases. There is no evidence that a bacterial cancer has occurred in horses.

Gastric carcinomas usually involve a combination between proliferation and destruction. This is the most aggressive form of gastric cancer in horses. The disease can spread directly to the liver, and then secondary spread to your lungs. The initial signs can be subtle. Horses with these symptoms are typically clinically ill. They may have recurrent colic, weight loss, and a dry, rough coat. A slightly anaemic horse could also be present.

These signs may be linked to other conditions, so they are often detected late in the course. A long endoscope is needed to diagnose. Sometimes, a gastric wash (containing both abnormal squamous and blood cells) can be used to diagnose the condition.

Carcinoma Treatment

Management of squamous cell carcinoma relies on either cryosurgery or surgical removal. Squamous cell carcinoma can be locally aggressive. If the eye or the area surrounding the eye has been infected, the prognosis for this condition is poor. Laser surgery has been tried in some cases, but it is not yet clear if this method will be successful long-term.

Radiation therapy is the best option for those sites that are too difficult to remove surgically or have poor margins.


Radiation therapy is the best option for treating all forms of squamous cells carcinoma of the skin in horses, as the tumour is sensitive to both beta and gamma radiation. Radiation treatment can cause damage to the eyes, so some tumors around the eyes cannot be treated.

Both radiation treatments use a radiation source that’s implanted at the lesion. Eyelid carcinoma is treated with gamma radiation using linear iridium 192wire. Beta radiation is provided by a Strontium 90plaque for lesions around the corner of the eyes and the third eyelid.

Radiation treatment has remarkable cosmetic results. Even in severe cases of eyelid damage, the process is quick and causes minimal complications.

Because beta radiation penetrates only 1-2mm into tissue, debulking may be required before radiation treatment. As this will help determine the best course, your vet should have any tissue taken to histopathology (under a microscope).


The best treatment for all types of carcinoma is surgical removal. However, the criteria for selecting this method are very strict. If the tumour has not been removed or the margins cannot be determined, surgical removal should be avoided.

For most cases of early penile cancer, surgery can be performed for third eyelid tumours and conjunctiva (linings of the eyelids).

Corneal carcinoma is a form of cancer that forms on the cornea. It can be removed surgically . This requires microsurgical techniques. The procedure involves removing a small area of the eye with 1-2mm around the tumor. The eye may burst if it is not done correctly.

Unless the tumour is very small or localised, eyelid tumors cannot be treated surgically. The extent of the tumor will determine whether the eyelid can still be saved or repaired. Inadequate eyelid function can cause severe pain and even loss of sight.

Penis amputation in horses suffering from penile carcinoma is curative. Recurrences are very rare if all of the tumour tissue has been removed. Smart surgery principles will be used to minimize cross contamination.

Large aggressive carcinomas can be removed temporarily, but the prognosis for this condition is not good.

With a good prognosis, vulvar and clitoral forms of carcinoma can both be removed surgically.


Although cryosurgery can’t be used for large lesions, it is possible to use it for smaller ones. Secondary effects of the freeze cycle may be severe so it is important to choose sites with care. Some oral and nasal lesions can be treated with cryosurgery, but only if the tumours are completely destroyed. Careful cryosurgery may be used to treat ulcerated lesions and bleeding areas. It is not recommended for all areas.

Intralesional and topical treatments

The type of treatment that is best suited for you will depend on the location of your lesion. Your veterinarian must administer all prescription medications.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!